INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY

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1 Supe J,, 2014; Volume 3 (2): INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY A PATH FOR HORIZING YOUR INNOVATIVE WORK ROLE OF HEAT OF HYDRATION IN ATTAINING EARLY STRENGTH GAIN OF CEMENT IN CONCRETE SUPE J 1, GUPTA MK 2 1. Asso. Prof., Civil Engg. Dept., SSIET, Durg. 2. Prof & Head Dept of Civil Engg. BIT, Durg. Accepted Date: 17/09/2014; Published Date: 01/10/2014 Abstract: When water is added to Portland cement heat is liberated. The rate and amount of heat produced is a function of cement composition. The heats of complete hydration is due to four major compounds of Portland cement, 3CaO.Si0 2, 2CaO.Si0 2, 3CaO.Al 2 O 3, and 4CaO.Al 2 O 3.Fe 2 O 3. The efficiency of heat of hydration of cement in concrete work s can be defined as bw 1 x100 where η = efficiency of heat of hydration of cement, sbw = setting time of cement snt with boiling water and s nt = setting time of cement with normal water used in preparing the mix. Ultimate strength of concrete is achieved after 28 days of. It is affected by many conditions, such as-: the proportion of the mixture and the influence of external agents and their temperatures etc. The objective of the paper is to carry out literature survey and study the influence of heat of hydration in gaining strength of concrete, when cured at various temperatures. The researchers have interesting results of their experimental investigations.the literature survey presented herein would provide platform to further investigate the influence of heat of hydration on concrete under different conditions like (i) to access the workability of concrete if boiling water is used while preparing the concrete, (ii) whether boiling water allows the concrete to gain 75-80% strength of targeted strength of 28 days in few days(iii) How the concrete in cold or hot weather can be beneficial to its mechanical performance w.r.t time, expense and constructive \ steps in normal temperature, etc. Research [15] shows that boiling water help in gaining strength at faster rate at initial stages (with in couple of days). Therefore authors in this paper carried out simple experimental study on behavior of strength when boiling water is used initially to prepare the mix and it is found that the results give answer to all the quarries raised above Keywords: Efficiency Of Concrete, Different Temperature Curing, Cement Hydration, Strength Corresponding Author: MR. JAYANT SUPE Access Online On: How to Cite This Article: PAPER-QR CODE Supe J, Gupta MK;, 2014; Volume 3 (2):

2 Supe J,, 2014; Volume 3 (2): INTRODUCTION When water is mixed with Portland cement, chemical reactions take place as a result of which heat is evolved. Under usual conditions of concrete construction, the heat is dissipated rapidly by radiation and temperature changes. In concrete structures of large mass, the heat is not readily radiated under hot climatic conditions therefore mass of the concrete may attain high temperatures. These temperature rises cause expansion while the cement is hardening, and may result in contractions and cracking when the eventual cooling to the surrounding temperature takes place. Also during construction in cold weather, special care must be exercised in the placing of concrete to ensure that the grout does not freeze. Since the reactions of hydration evolve heat, they are capable of producing a degree of auto-protection against an outside temperature which might otherwise cause freezing or an inhibition of the hardening process. For each of the above conditions of construction it is desirable that information be available on the total quantity of heat which may be evolved from the cement and especially on the rate at which this heat is liberated. The hydration process affects the quantity and rate of heat evolution. The net heat evolved at any given age of setting of concrete is due almost entirely to the hydration which has taken place up to the time of setting. The water causes the hardening of concrete through a process called hydration. It is a chemical reaction in which the major compounds in cement forms chemical bonds with water molecules and become hydrates or hydration products. The role of water is important because too much water reduces concrete strength, while too little will make the concrete unworkable. Portland cement is a hydraulic cement which derives strength from chemical reactions between the cement and water. Cement consists of the following major compounds: o o o o Tricalcium silicate, C 3 S Dicalcium silicate, C 2 S Tricalcium aluminate, C 3 A Tetracalcium aluminoferrite, C 4 AF o Gypsum, CSH 2 Chemical reactions during hydration When water is added to cement, the following series of reactions occur: 95

3 Supe J,, 2014; Volume 3 (2): The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat: Tricalcium aluminate + gypsum + water = ettringite + heat C 3 A + 3CSH H = C 6 AS 3 H 32, + H = 207 cal/g.( i ) Ettringite consists of long crystals that are only stable in a solution with gypsum. The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat: Tricalcium silicate + water = calcium silicate hydrate + lime + heat 2C 3 S + 6H = C 3 S 2 H 3 + 3CH, + H = 120 cal/g..( ii ) The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue. Once all the gypsum is used up as per reaction (i) the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals: Tricalcium aluminate + ettringite + water = monosulfate aluminate hydrate 2C 3 A + 3C 6 AS 3 H H = 3C 4 ASH 18,..( iii ) The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back into ettringite, whose crystals are two-and-a-half times the size of the monosulfate. It is this increase in size that causes cracking when cement is subjected to sulfate attack. The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat: Dicalcium silicates + water = calcium silicate hydrate + lime 2C 2 S + 4H = C 3 S 2 H 3 + CH, + H = 62 cal/g..( iv ) Like in reaction (ii), the calcium silicate hydrates contribute to the strength of the cement paste. This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of C 2 S to the strength of the cement paste will be slow initially. This compound is however responsible for the long-term strength of portland cement concrete. 96

4 Supe J,, 2014; Volume 3 (2): When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate C 3 S is responsible for most of the early strength (first 7 days). Dicalcium silicate C 2 S, which reacts more slowly, contributes only to the strength at later times. The equation for the hydration of tricalcium silicate is given by :- Tricalcium silicate + Water Calcium silicate hydrate + Calcium hydroxide + heat 2Ca 3 SiO 5 + 7H 2 O 3CaO. 2SiO 2. 4H 2 O + 3 Ca(OH) kJ..( v ) Upon the addition of water, tri-calcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The ph quickly rises to over 12 because of the release of alkaline hydroxide (OH - ) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved. The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier's principle). The evolution of heat is then dramatically increased. The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower. Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate: Dicalcium silicate + Water Calcium silicate hydrate + Calcium hydroxide +heat 2Ca 2 SiO H 2 O 3 CaO. 2SiO 2. 4H 2 O + Ca(OH) kj..( vi ) 97

5 Supe J,, 2014; Volume 3 (2): Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown below as a function of time. Fig.(1) The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage. The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. This is particularly important for the construction trade who must transport concrete to the job site. It is at the end of this stage that initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tri-calcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and un-hydrated silicates are present. Fig(1): Rate of heat evolution during the hydration of Portland cement In all above reactions with different cement compounds it is found that lot of heat energy is produced which is utilized in raising the temperature of water molecules that are in ground state, when mixed at normal temperature. Instead if boiling water is used to prepare the mix the water molecules will be in excited state, then this heat energy evolved would be utilized in allowing the water molecules to form stable bonds with cement compounds and thus initial setting time of cement paste will accelerate ( as found experimentally in Table no.2 ), and hence initial strength gain in concrete will be faster, as it has been found in Table no.1, concrete cubes has been cured in boiling water in early ages for 3½hrs after in normal water for 23 hrs. Strength of Concrete The concrete strength increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more 98

6 Supe J,, 2014; Volume 3 (2): water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph Fig.(2). Fig.(2): A plot of concrete strength as a function of the water to cement ratio. Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability. Importance of Concrete Temperature - Concrete temperature is directly related to the development of strength. Fresh concrete can be damaged when exposed to very low or very high temperatures. Many agencies have specifications that regulate the fresh concrete temperature upon delivery, as well as maximum and minimum temperatures during critical early ages. An effect known as crossover relates the long-term strength of concrete to the temperature during the period. As the graphs below show, the same mix will gain strength quicker at high temperatures, but at later ages the strength curves will crossover leaving the concrete cured at the highest temperature with the lowest final strength. This is important because most design strength criteria assume concrete to gain in strength after the standard 28 days. 99

7 Supe J,, 2014; Volume 3 (2): Literature Survey on Concrete strength gain Fig.(3): A plot of concrete strength vs time Concrete strength increase with age as moisture and a favorable temperature is present for hydration of cement. The durability of concrete is affected by a number of factors including its permeability, porosity and absorptive. Well cured concrete can minimize thermal, plastic & drying shrinkage cracks, making concrete more water tight, thus preventing moisture and water borne chemicals from entering into the concrete and thereby increasing its durability. Concrete that is allowed to dry out quickly undergoes considerable early age shrinkage. Inadequate contributes to weak and dusty surfaces having a poor abrasion resistance. Material properties are directly related to their microstructure. Curing assists the cement hydration reaction to progress steadily and develops calcium silicate hydrate gel, which binds the aggregates leading to a rock solid mass, makes the concrete denser, decreases the porosity and enhances the physical and mechanical properties of concrete. 2(CaO) 3 (SiO 2 )+6H 2 (CaO) 3 (SiO 2 )3H 2 O + 3Ca(HO) 2..( vii ) C-S-H gel 2(CaO) 2 (SiO 2 )+4H 2 O (CaO) 3 (SiO 2 ) 2 3H 2 O + Ca(HO) 2..( viii ) Right Time to Cure Concrete C-S-H gel After concrete has been placed in its final position and during the initial set, bleed water rises to the concrete surface as plastic settlement occurs. During this period, if the rate of evaporation of bleed water is greater than the rising water, plastic shrinkage of the concrete 100

8 Supe J,, 2014; Volume 3 (2): occurs. Initial mist is necessary to keep the surface moist to prevent the surface from drying out. Between initial set and final set, intermediate would be needed if the finishing is complete prior to final set. This may be in the form of a barrier which prevents the loss of moisture from the concrete surface. e.g. covering the concrete surfaces with plastic sheets, waterproof paper, etc. After final set, meticulous will have to be done as per the procedures selected. e.g. water methods- Ponding, Misting, wet coverings with hessian cloth, Impermeable membrane, Curing compounds, etc. Duration of Curing The duration of of concrete depends on the grade & type of cement, mix proportion, desired concrete strength, shape and size of the concrete member and environmental & exposure conditions. The duration may vary from few days to a month. IS-456:2000 provisions for duration of Curing (Indian Standard-Plain & Reinforced concrete-code of Practice, 4th revision, page 27). Secondary reaction in case of blended cement : 1. Portland Pozzolana Cement ( PPC ) OPC + Water C-S-H gel + Ca(OH) 2..( ix ) (Primary gel) Ca(OH) 2 + SiO 2 C-S-H gel..( x ) (Secondary gel) 2. Portland Slag Cement ( PSC ) OPC + Water C-S-H gel + Ca(OH) 2..( xi ) (Primary gel) GGBS + water Activated by Alkali s C-S-H + SiO 2..( xii ) (Primary gel) (Silicates) 101

9 Supe J,, 2014; Volume 3 (2): Ca(OH) 2 + SiO 2 C-S-H gel..( xiii ) (Silicates) (Secondary gel) Exposed surfaces of concrete shall be kept continuously damp or in a wet condition by ponding or by covering with sacks, canvas, hessian or other similar material and kept continuously wet for at least 7 days from the date of placing, in case of Ordinary Portland Cement (OPC) and at least 10 days when mineral admixtures or blended cements are used. In case of concrete where mineral admixtures or blended cements are used, it is recommended that the above minimum periods may be extended to 14 days, for assisting the secondary reaction. C-S-H: main phase of hydrated Portland Cement- C-S-H (calcium-silicate-hydrate) is the main phase of hydrated Portland and blended cements and is responsible for strength development and hydraulic behavior. Its composition is normally defined by its calcium/silica ratio. Several authors derived solubilities from synthetic C-S-H [1],[2],[3],[4],[5],[6] and the overall picture is consistent. The maximum Ca/Si ratio of C-S-H in hydrated cement pastes is variously claimed to lie between 1.5 and 1.9 in portlandite saturated systems and, due to phase rule restrictions, C-S-H in Portland cements and blended cements which contain excess portlandite, will have as a first approximation a nearly constant composition ~ CaO SiO2 nh2o, where n is largely dependent on the humidity of the hydrated cement paste. Several thermodynamic models [7][8][9] have been developed to describe the solubility properties of C-S-H. According to Kulik and Kersten [10] an ideal solid solution model is sufficient to describe the solubility behaviour of C-S-H, as solid solution is continuous and no miscibility gaps at relevant Ca/Si-ratios are reported. Tobermorite-type C-S-H (Ca/Si=0.83), a jennite-type C-S-H (Ca/Si=1.67) and amorphous silica were used as end members of two different solid solution series, as defined by Lothenbach et al. [11], who gave a detailed description of the derivation of thermodynamic data for the C-S-H end members; related data were compiled and recalculated solubility of C-S-H which agrees reasonably well with literature values. According to Kulik and Kersten [10] one solid solution (tobermorite-jennite) describes the Ca/Si range 0.83 (Ca/Si)C-S-H 1.67 while a second solid solution (amorphous silica - tobermorite) is used to describe the range 0 (Ca/Si)C-S-H However, C-S-H gel coexists with a silica species with low calcium content and a composition close to amorphous silica at initial Ca/Si ratios below ~ , indicating either a miscibility gap in the second solid solution series or that it has a sharp cut-off at a critical Ca/Si ratio. 102

10 Supe J,, 2014; Volume 3 (2): Experimental work with use of boiling water for of concrete - Authors have already reported in their study 15 that boiled water (BWC) results into 75-80% gain in strength in concrete as apparent from following tables: Normal Curing ( NC) Boiling Water Curing (BWC) Table 1 Compressive strength results after 23hrs & 28 Days Normal Curing OPC 43 Grade Cement M20 23hrs M25 23hrs M30 23hrs M20 Normal M25 Normal M30 Normal PSC Cement M20 23hrs M25 23hrs M30 23hrs M20 Normal M25 Normal M30 Normal PPC Cement 103

11 Supe J,, 2014; Volume 3 (2): M20 23hrs M25 23hrs M30 23hrs M20 Normal M25 Normal M30 Normal Experimental Study to determine efficiency of heat of hydration reaction- Vicat s Needle apparatus has been used to find initial setting time, with cement paste prepared from: OPC, PPC and PSC types of cement. The paste were prepared with normal water and also with boiling water. It was found that setting time of all types of cements decreased drastically due to addition of boiling water. The results are tabulated below: The efficiency of heat of hydration of cement in concrete work can be defined as :- s bw 1 x100 snt where η = efficiency of heat of hydration of cement, s bw setting time of cement with boiling water and s nt setting time of cement with normal water used in preparing the mix. Table 2 Initial Setting of different cement No. Cement Type Time S bw min Time S nt min Efficiency 1 OPC 43 Ultra Tech % 2 PPC Ultra tech % 3 PSC Ultra Tech % 104

12 Supe J,, 2014; Volume 3 (2): Samples of Boiling water mix & Normal water mix tested on Vicat s apparatus CONCLUSION Literature survey shows that each of the compounds in cement has a role to play in the hydration process. By changing the proportion of each of the constituent compounds in the cement, it is possible to make different types of cement to suit several construction needs and environment. Earlier authors have reported in their study 15 that boiling water (BWC) in concrete works results into 75-80% gain in initial stages of compressive strength in concrete in case of OPC cement Table 1. Initial compressive strength of PPC and PSC cement are lower as there is presence of Pozzolana and Slag mixed respectively, which reduces early setting of cement ultimately reducing early strength gain of concrete even in Boiling Water Curing. Also it has been found workability of concrete mix also reduces if boiling water is used at time making concrete mix in all types of cement. Encouraged by above test results the it was thought of using boiling water to study increase in efficiency of heat of hydration reaction. Therefore Vicat s needle test method is adopted to study the initial setting of cement due to role of heat of hydration. Samples have been prepared with different types of cement and their setting time are determined. The results reported in Table No. 2,shows that the setting time cement reduces drastically in case OPC,PPC and PSC respectively, ultimately increasing the efficiency of concrete if Boiling Water Curing is done to concrete in early ages. REFERENCES 1. Flint, E.P.; Wells, L.S.: Study of the system CaO-SiO2-H2O and of the reaction of water on the anhydrous calcium silicates. Journal of Research of the National Bureau of Standards, A. Physics and. Chemistry. 1934, 12, Fujii, K.; Kondo, W.: Heterogeneous equilibrium of calcium silicate hydrate in water at 30 C. Journal of the Chemical Society, Dalton transactions, 1981, issue 2,

13 Supe J,, 2014; Volume 3 (2): Greenberg, S.A.; Chang, T.N.: Investigations of the colloidal hydrated calcium silicates. II, Solubility relationships in the calcium oxide-silica-water system at 25 C. The Journal of Physical Chemistry, 1965, 69, Roller, P.S.; Ervine, G.: The system calcium oxide-silica-water at 30 C. The association of silicate ion in dilute alkaline solution. Journal of the Chemical Society, 1940, 62, Suzuki, K.; Nishikawa, T.; Ito, S.: Formation and carbonation of C-S-H in water. Cement and Concrete Research, 1985, 15, Taylor, H.F.W.: Hydrated Calcium Silicates, Part I: Compound formation at ordinary temperatures. Journal of the Chemical Society, 1950, notes 726, Berner U.R.: Modelling the incongruent dissolution of hydrated cement minerals. Radiochimica Acta, 1988, 44/45, Fujii, K.; Kondo, W.: Estimation of thermochemical data for Calcium Silicate Hydrate (C-S- H). Journal of the American Ceramic Society, 1983, 66, C 220 C Gartner, E.M.; Jennings H.M.: Thermodynamics of calcium silicate hydrates and their solutions. Journal of the American Ceramic Society, 1987, 70, Kulik D.A. and Kersten M.: Aqueous solubility diagrams for cementitious waste stabilisation systems: II, End-member stoichiometries of ideal calcium silicate hydrate solid solutions. Journal of the American Ceramic Society, 2001, 84, Lothenbach, B.; Matschei, T.; Möschner, G.; Glasser, F.P.: Thermodynamic modeling of the effect of temperature on the hydration and porosity of Portland cement. Cement and Concrete Research, 2008, 38, Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc., 15. Jayant Damodar Supe* & Dr. M.K.Gupta**, Predictive Model of Compressive Strength for Concrete In-Situ, IJSCER ISSN , Vol. 3, No.1, February 2014 pages